The present invention relates generally to sound reproduction equipment, and particularly to speaker enclosures including transmission line acoustic coupling and to audio reproduction equipment tuned relative to a specific listening environment.
Audio reproducing systems continue to evolve toward higher quality sound reproduction. Inherent non-linearity, i.e., variation in sound energy as a function of sound wavelength, continues to improve through research and development. From audio recording to audio reproduction, vast improvement in quality of equipment benefits the discriminating listener. Unfortunately, challenge remains in the context of distortion and frequency response for most audio equipment, especially at low frequency or bass wavelengths. Even highly advanced equipment suffers at extreme low frequencies in faithfully reproducing a linear sound presentation.
A high quality musical sound wave emerges from a loud speaker diaphragm coupled acoustically to a listening room, and a corresponding inverse-phase sound wave emerges from the rear of the speaker diaphragm. This rear-traveling sound wave, upon eventually being coupled to the surrounding air mass, introduces non-linearity in the otherwise high quality sound provided within the room by the front-traveling sound wave. Solutions have evolved, but not always proportionately for sound quality improvement in relation to expense.
A traditional cone diaphragm speaker pushes air forward out of the speaker enclosure in producing sound waves within a listening room Sound waves emanating from the front and rear of the speaker diaphragm are complimentary, i.e., 180 degrees in phase relationship. Accordingly, coupling the forward traveling and rearward traveling sound waves within a common listening room can introduce non-linearity in sound presentation due to sound wave interference and cancellation. Ideally, such rear-traveling sound waves couple to a separate listening chamber, thereby avoiding sound wave interference and cancellation. For example, mounting speaker diaphragms within walls sends front-traveling waves to a first listening room and rear-traveling waves to a second listening room. Unfortunately, elaborate wall-mounted speaker systems are impractical for most listeners.
The traditional mechanism delivering sound presentation within a listening room is a speaker within an enclosure. The speaker diaphragm couples directly at its front surface to the listening room, and at its rear surface to the interior of the enclosure. Unfortunately, high quality sound reproduction requires venting or release from the enclosure of the rear-traveling sound waves, i.e., eventually the rear-traveling sound waves must exit the enclosure. The rear-traveling sound waves, upon emanating from the enclosure, preferably introduce little or no interference or sound wave cancellation relative to the front-traveling sound waves.
Acoustic transmission line speakers manage rear-traveling sound waves within a speaker enclosure. Generally, a transmission line speaker enclosure provides acoustic coupling from the rear surface of the speaker diaphragm to the listening room along a transmission line or chamber of given length and cross sectional area. Acoustic transmission line length is a function of the wavelength of a particular sound frequency, e.g., speaker resonance. Cross sectional area corresponds to the effective surface area of the sound source, e.g., effective surface area of the speaker diaphragm.
A variety of acoustic transmission line speakers are known and commercially available. Unfortunately, due to the significant chamber length required in most acoustic transmission line speakers, i.e., those directed to management of very low frequency sound waves, acoustic transmission line speakers have evolved into large and massive structures. The acoustic transmission line can be "folded" or routed within the enclosure in a labyrinth to establish the required length within an overall box-like shape. Panels, typically wood, within the enclosure form the required acoustic transmission line or chamber with appropriate cross sectional area therealong. To resist deformation of the panels in response to sound pressure within the acoustic transmission line, such panels must be of sufficient structural integrity, i.e., thickness, to maintain rigidity against sound wave pressure. The combination of thick panel structures forming the acoustic transmission line as a folded labyrinth within the speaker enclosure results in massive and large overall volume speaker enclosures.
The subject matter of the present invention addresses this aspect of transmission line speaker enclosures by providing a transmission line speaker enclosure having an acoustic transmission line of appropriate length and cross section, but not requiring a large volume, massive speaker enclosure structure.
A reverberating sound wave, established by surrounding walls, floor, and ceiling, also brings interference relative to other sound waves within the listening room. This interference introduces non-linearity in the otherwise high quality sound provided at the loud speaker. Sound absorbent material in the listening room and elaborate tuning schemes attempt to minimize such non-linearity, but such methods and apparatus do not always proportionately improve sound quality in relation to the magnitude of expense required.
Cavity resonance in a listening room provides a significant source of reverberation interference degrading a high quality sound presentation. Room cavity resonance operates at a given fundamental frequency and associated harmonic frequencies. Across a range of typical room sizes, the fundamental resonate frequency falls in an audible frequency band. Due to cavity resonance, sound energy at the fundamental frequency does not dissipate as do other sound frequencies. Sound pressure, developed at the fundamental and harmonic frequencies, tends to build. The listener perceives a relatively louder sound at the resonant and harmonic frequencies. In other words, sound pressure tends to build excessively at the fundamental and harmonic frequencies within a given listening room and becomes, for the discriminating listener, an annoying departure from linear sound presentation.
Unfortunately, cavity resonance for a given listening room varies as a function of air density, room furnishings, or barometric conditions. Predicting narrow band cavity resonance in a given listening room becomes impossible. Cavity resonance can be as narrow as one hertz (Hz) in some listening rooms. Accordingly, an attempt to anticipate cavity resonance and filter such narrow fundamental frequency bands fails due to the narrow and unpredictable character of the fundamental and harmonic resonant frequencies.
The subject matter of the present invention addresses this aspect of audio reproduction by providing an apparatus and method for eliminating cavity resonance from an audio presentation.